The invention is directed to a process for the production of ceramic fuel pellets for nuclear reactors which contain the fissionable and/or breeder material in oxide or carbide form by heating to a temperature above 1300.degree. C. the fuel crude molded articles compacted to 30 to 60% of theoretical density at room temperature, compacting in a die and expelling from the die.
The fuel elements for light water reactors (LWR) and quick sodium cooled reactors (SNR) contain the fissile and/or fertile material predominantly in the form of sintered pellets of uranium oxide or a mixture of uranium oxide and plutonium oxide. The plutonium is preponderantly used in SNR reactors but also is employed in limited amounts in the LWR reactors in place of U.sub.235. Besides fuel oxides there are also employed in SNR reactors fuel carbides (K. Kummerer, in KfK-1111 (1969), EUR-4315d (1969) Contribution I). The high susceptibility to corrosion of the carbide toward moisture, however, excludes the carbide as fuel in water or steam cooling.
While the LWR fuel pellets generally have a height of about 12 mm, a diameter of about 10 mm and a weight of about 10 grams, the corresponding data in the normal case for SNR tablets are 6 mm, 5 mm and 2.5 grams. Oxidic fuel pellets are customarily produced from uranium oxide powder (UO.sub.2) of from mixed powder oxides UO.sub.2 and PuO.sub.2 by pressing, sintering and circular grinding. The oxides needed for this must inter alia particularly have sintering properties which can only be attained through expensive processes of production (e.g. see German Pat. No. 1,592,468, German AS No. 1,592,477 or German OS No. 2,623,977 and related Borner U.S. Pat. No. 4,152,395).
In the production of carbidic fuel there is first normally reacted uranium oxide powder, or a mixture of uranium-plutonium oxide powder with carbon by carboreduction to form uranium carbide or uranium-plutonium carbide and subsequently the reaction product finely ground to the sinterable powder. From this analogous to the oxidic field there are prepared pellets by pressing, sintering and grinding.
Isaacs U.S. Pat. No. 3,236,922 shows mixing which free uranium powder, carbon powder (e.g. graphite) and plutonium monocarbide. This mixture is cold compressed, sintered at 1000.degree.-1300.degree. C., the temperature allowed to go above the melting point of uranium metal and then the sintering completed to form mixed uranium monocarbide-plutonium monocarbide.
Furthermore, there have been experiments to produce oxidic and carbidic fuel pellets by hot pressing at the required temperatures of more than 1300.degree. C. there cannot be solved industrially the material problems for punches and die. The tolerances in dimensions produced thereby nevertheless required an expensive subsequent processing. Therefore, this process has not acquired any industrial significance.
Besides it is known to prepare carbidic fuel cylinders by casting with subsequent working on all sides. With the small dimensions of the SNR pellets, however, this process is likewise uneconomical. The tight casting structure is unsuited for high burn-ups. For these reasons, this process also did not come to practical use.
The previous process for the production of sintered fuel pellets has a series of disadvantages, especially high demands are placed on the quality of the pressing powder.
The geometric density of the fuel pellets after the pressing is small and at a work tool maximum applied pressure of 500 to 1000 MN/m.sup.2 (MN is the abbreviation for Mega-Newton) limited to less than 79% of the theoretical density. Consequently as expensive subsequent densification through sintering is required. In order to guarantee the quality of the sintered pellets high requirements are placed on the pressing powder in regard to the purity and sinterability which can only be fulfilled with expensive processes.
In the handling of plutonium because of its radiation danger and toxicity there must be provided special safety precautions which are very expensive and therefore raise the manufacturing costs considerably. The disadvantage in a Co precipitation from the production of uranium to plutonium to the UO.sub.2 /PuO.sub.2 consists of the expansion of the expensive plutonium safety precautions also to the main component uranium, since already at the beginning of this process it must be mixed with plutonium.
The pyrophoric and hydrolytically sensitive properties of the sinterable carbide powder create in the production of uranium-plutonium pellets a further expense. For this reason extremely high requirements are placed on maintaining a highly pure atmosphere in the line of boxes. By these additional precautions the costs of manufacturing the fuel element are again increased. For these reasons the carbidic fuel, although known for many years, was used for special purposes but was not included broadly.
The presses used in the previous process for the production of fuel pellets with great molding force and the sintering furnace about 13 meters long require a large amount of space. Especially in the processing of plutonium containing fuel the boxing in of large devices is expensive and is connected with high maintenance and upkeep costs.
The diameter tolerances of the fuel pellets is .+-.25.mu.. In order to keep this all pellets after the sintering must be circular ground to target dimensions. The decrease in weight in the grinding is 2 to 4%. Besides there cannot be avoided a scrap of about 10% through breakage of edges and through spalling. Above all the processing of the plutonium containing grits has proven extremely difficult and is associated with a very high expense.
The sintered fuel pellets in the reactor are frequently inclined to post sintering. This post densification increases the fission gas pressure inside the pellets and besides contributes to increasing the heat retarding gap between the pellets and the cladding tube and consequently the heat transfer is deteriorated.